Abstract
Magma is a viscoelastic fluid that can support fracture propagation when local shear stresses are high, or relax and flow when shear stresses are low. Here we present experiments to confirm this using synthetic and natural magmatic liquids across eruptive conditions and use Maxwell’s linear viscoelasticity to parameterize our results and predict the maximum stresses that can be supported during flow. This model proves universal across a large range of liquid compositions, temperatures, crystallinity and rates of strain relevant to shallow crustal magma ascent. Our results predict that the 2008 Volcán Chaitén eruption resided in the viscous field at the onset of magma ascent, but transitioned to a mixed viscous-brittle regime during degassing, coincident with the observed combined effusive-explosive behaviour during dome extrusion. Taking a realistic maximum effusive ascent rate, we propose that silicic eruptions on Earth may straddle the viscous-to-brittle transition by the time they reach the surface.
Highlights
Magma is a viscoelastic fluid that can support fracture propagation when local shear stresses are high, or relax and flow when shear stresses are low
An apparent dichotomy exists in volcanic eruptions on Earth between events that are dominantly effusive—producing lavas—and those that are explosive in character—
Maxwell proposed the simplest model of how elastic shear stresses are stored or relaxed in fluids; such that relaxation occurs over a characteristic time λr = μ/G∞ where μ is the liquid viscosity (Pa.s) and G∞ is the elastic shear modulus (Pa)[1,28,29]
Summary
Magma is a viscoelastic fluid that can support fracture propagation when local shear stresses are high, or relax and flow when shear stresses are low. This results in a critical Weissenberg number as Wi 1⁄4 λ′r=λ′, which is equivalent to the Wi for singlephase liquids, and permits all data to be plotted together in a master regime diagram for multiphase and single-phase magmas where the breaking point Wic is at a constant position (Fig. 4).
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